Field of the Invention
The present invention relates to a magnetic recording media, such as magnetic recording tapes, magnetic recording disks, or magnetic recording cards, and to a magnetic recording system. More particularly, the present invention relates to thin-film media suitable for high density magnetic recording and a magnetic recording system using the media.
As electric computers have become smaller and the speed thereof has been increased, there has been a great demand for higher capacity magnetic disk apparatuses and other external storage apparatuses and higher access thereto. In particular, magnetic disk recording apparatuses are information recording apparatuses suited for higher density and higher speed, and the demand for these apparatuses is getting even higher. As for recording media used in magnetic disk apparatuses, media with magnetic oxide powders coated on the substrate, and thin-film media having thin films of magnetic metals sputtered on the substrate have been developed. This thin film media, as disclosed in, for example, Japanese Patent Laid-Open Nos. 58-7806 and 60-111323, have a higher content of magnetic material in the magnetic recording layer than that of coated media, and therefore it is suitable for recording and reproduction at high density.
A magnetoresistive (hereinafter abbreviated as MR) head having improved read sensitivity over that of a conventional inductive head has been developed by using an MR sensor in the reproduction section of the magnetic head, as disclosed in for example, Japanese Patent Laid-Open Nos. 62-40610 and 63-117309. Since a satisfactory S/N ratio can be obtained even if the area of recording bits is small when this head is used, it is possible to remarkably improve the recording density of media.
Aluminum alloys, glass, ceramics or organic resins are used for substrates of thin film media. An Ni--P-plated layer or an anodic oxide film, having a thickness of, for example, approximately 10 .mu.m, is formed on the surface of a disk substrate in order to improve molding characteristics such as hardness or flatness, or magnetic characteristics. On the surface of the substrate of the media, there are formed very small grooves, as disclosed in U.S. Pat. No. 4,735,840, Japanese Patent Laid-Open Nos. 61-29418, 62-146434, and 63-121123, the Journal IEEE Trans. Magn., Vol. MAG-22 (5), p. 579, 1966, or IEEE Trans. Magn., Vol. MAG-23 (5), p. 3405, 1987, substantially in the recording direction, for example, substantially circumferentially for the disk media. The grooves, called texture, are formed by cutting the surface nearly circumferentially with abrasive grains. Hitherto, the average roughness factor (Ra) of the groove was in a range from approximately 2 nm to 10 nm. When such texture is formed, the frictional force of the magnetic head contacting the media decreases, so that the problem of the head adhering to the surface of the media during a contact start/stop (hereinafter abbreviated as CSS) operation is aboided. Further, if the average roughness factor of the groove, the thickness of an underlayer, or film forming conditions of the media are changed appropriately, the magnetic characteristics of magnetic films measured by applying a magnetic field in the recording direction, for example, coercivity Hc, a residual magnetic susceptibility Br, coercive squareness S*, or magnetic anisotropic energy K measured by applying a magnetic field within the surface of the substrate while rotating a specimen within the surface of the substrate, are more improved than when no texture is formed, and the signal to noise ratio (S/N ratio) of the read output and resolution are improved. Further, there is a problem called modulation in which nearly circumferential magnetic characteristics becoming nonuniform within the surface of the media depending upon the heating temperature during the formation of the media and upon the transfer method, causing a read output to vary within the surface of the media. However, if the depth of the groove, the composition of the underlayer, or the film forming conditions are changed appropriately, nearly circumferential magnetic characteristics are made uniform within the surface of the media. As a result, modulation is suppressed. The above effect has been observed.
FIG. 1 is a longitudinal, sectional view of a thin-film magnetic recording medium, illustrating the relationship between the medium and the MR head. In FIG. 1 reference numeral 11 denotes a magnetic head, reference numeral 17 denotes a substrate formed from Al--Mg alloy, and reference numeral 15 denotes non-magnetic plated layers formed from Ni--P, whose surface was textured to form grooves 16. Reference numerals 14, 13 and 12 denote a underlayer, a magnetic layer and a protective layer formed on the textured Ni--P plated layer.
To improve the recording density of a thin film medium, it is important to make the spacing (hereinafter abbreviated as the head flying height) between the magnetic head and the recording medium as small as possible. This is because a sharp magnetic-field distribution is formed within the media during recording, and magnetic fluxes from the medium can be efficiently detected during reading, thereby suppressing a loss in the read output. However, if the head flying height is decreased in a textured medium, the incidence of the contacting of the magnetic head with the medium increases more than that of a flat substrate having no texture. A detailed examination shows that the cause for the above is that irregular, very small projections are unavoidably formed on the surface of the medium as the result of texturing, and if the head flying height is decreased, the projections contact the magnetic head. A method of removing projections on the surface of the substrate by a polishing process, to reduce the incidence in which the head contacts the medium, is disclosed in Japanese Patent Laid-Open No. 1-162229. However, this method has the problem that the magnetic characteristics and anisotropic energy of the magnetic film measured by applying a magnetic field in the recording direction less than before the projections are polished, the S/N ratio decreases, and modulation is caused.
There is another problem in that, when the depth of the groove is great, the uniformity and S/N of servo signals which have been recorded previously in the medium, necessary for the head to follow tracks in which information is recorded, are worse than in a flat substrate with no texture, making it impossible to increase the density of the tracks.
Making the depth of the groove small is effective to solve the problem of head flying characteristics and deterioration of servo signals. However, as described in the Journal IEEE Trans. Magn., Vol. MAG-23 (5), p. 3405, 1987, if the depth of the groove decreased, the problem arises that the magnetic characteristics of the magnetic film measured by applying a magnetic field in the recording direction. Here, the orientation ratio of coercivity Hc in the recording direction is defined as {Hc(.theta.)-Hc(r)}/{Hc(.theta.)+Hc(r)} on the basis of coercivity Hc(.theta.) measured by applying a magnetic field in the recording direction along the recording truck, and coercivity Hc(r) measured by applying a magnetic field, within the surface of the media, substantially perpendicularly to the recording direction across the recording truck.
The above-described orientation ratio of Hc is closely related to the recording and reproduction characteristics of the medium. The results of detailed experiments show that the orientation ratio is preferably positive (greater than 0) to obtain S/N ratio of read output of 3 or more, and more preferably 0.1 to 0.7 to obtain S/N of 4 or more when the linear recording density is 50 k BPI (Bits Per Inch) and the track density is 3 k TPI (Tracks Per Inch). Also, it has been shown that in-plane magnetic anisotropic energy measured by rotating a specimen within the surface of the media while applying a magnetic field parallel to the surface of the media is preferably 3.times.10.sup.4 J/m.sup.3 to 5.times.10.sup.5 J/m.sup.3. However, in the conventional art, decreasing the size of the groove in order to control the orientation ratio of coercivity Hc within the above-described range has not been known hitherto, and the average roughness factor of the groove needs to be greater than 3 nm.